1. Field of the Invention
This invention relates to a process and apparatus for generating electricity from coal using pressurized fluidized bed combustion for converting coal to a fuel gas.
2. Description of the Prior Art
The electric utility industry is constantly seeking new electricity generation technology which can operate with substantially improved efficiencies and improved environmental performance. In conventional power plants, solid fuel is combusted under atmosoheric pressure and steam is generated producing electricity at an overall efficiency of less than 40%. The resulting flue gas is cleaned and subsequently exhausted into the atmosphere.
One power generation scheme under development for increasing the efficiency and improving the environmental performance of electricity generating plants is the integrated gasification combined cycle (IGCC) in which the solid fuel, typically coal, is first gasified under pressure. The resulting fuel gas is cleaned, burned in a combustion chamber, and then expanded in a gas turbine. Such systems, which can employ a wide variety of gasification technologies, are taught by Kurkela, E. "Gasification Combined Cycle Power Generation-Process Alternatives", Proceedings of VTT Symposium 83-Fluidized Bed Combustion and Gasification Systems, Espoo, Finland, 37-52, 1987.
An alternative to gasification of the solid fuel to produce a fuel gas as in the IGCC power generation system is pressurized fluidized bed combustion in which the solid fuel is fed to a pressurized carbonizer, producing a low Btu fuel gas and char. After cleaning, the fuel gas is burned in a combustor along with flue gas produced from residual char combustion to produce energy which drives the gas turbine. Such a system is taught by Robertson, A. et al., "Second Generation PFB Combustion Plants," paper presented at GEN-UPGRADE 90 International Symposium, Washington, D.C., March 1990.
Both the integrated gasification combined cycle system as well as second generation pressurized fluidized bed combustion systems with topping cycles have achieved coal to electricity conversion efficiencies up to 45% using advanced turbine technology with gas inlet temperatures of approximately 2300.degree. F. However, due to the nature of the required processing steps which are temperature limited due to the amount of process heat which is extracted from the system and used for steam generation prior to delivery of the gas and subsequent extraction of power from the gas turbine, the efficiency of these systems is generally limited. For example, the gasification system which propels the IGCC system is operated in the range of 1800.degree.-2000.degree. F. The resulting fuel gas at these temperatures must be cooled to approximately 1000.degree. F. for removal of sulfur compounds prior to combustion in the turbine combustor. The heat recovered from cooling the gas is used in generating steam in a relatively less efficient steam cycle. Similarly, in second generation pressurized fluidized bed combustion systems, the residual char is combusted at 1500.degree.-1600.degree. F. which combustion temperature is maintained by extracting heat from the system and raising steam for a relatively less efficient steam cycle.
There are three main types of gasifiers suitable for use in an IGCC system: fixed bed gasifiers, fluidized bed gasifiers, and entrained-bed reactors.
One of the best known fixed-bed gasifiers is the Lurgi gasifier in which lump coal is pressurized in a multiphase lockhopper and introduced into the upper zone of a reactor. A mixture of air or oxygen and steam is fed to a lower portion of the gasifier. The coal flows slowly downward through drying, pyrolizing, reduction and combustion zones in the gasifier, producing a fuel gas and ash. In the most well known fixed-bed process for combined cycle application, the British Gas slagging Lurgi, the temperature of the lower gasifier zone is maintained above the ash melting temperature.
In an entrained-bed gasifier, finely pulverized coal and air or oxygen are introduced into a reactor where gasification occurs at a temperature greater than 1200.degree. C. In most instances, the fuel is fed into the reactor as a slurry.
In fluidized bed gasifiers, fuel is introduced into the reactor and typically gasified at temperatures between about 750.degree.-1000.degree. C. Such a pressurized fluidized bed process is taught by U.S. Pat. No. 4,315,758.
See also U.S. Pat. No. 4,872,886, which teaches a two-stage upflow process for coal gasification in which a slurry of particulate carbonaceous material in a liquid carrier and a stream of oxygen-containing gas is burned in a fired horizontal slagging reaction zone, or first stage reactor, producing steam, vapor from the liquid carrier, char and gaseous combustion products, all of which are contacted in an unfired vertical second stage reactor with a second increment of slurry of particulate carbonaceous solids in a liquid carrier to yield steam, vapor from the liquid carrier, synthesis gas and char entrained in the gaseous effluent Slag produced in the first stage reactor is removed through a tap hole in the bottom of the reactor.
U.S. Pat. No. 4,338,283, and related U.S. Pat. No. 4,828,486, teach a fluidized bed combustor in which the combustion chamber and a regeneration chamber for regeneration of a desulfurizing agent are both contained within a single hollow body, the hollow body being vertically partitioned by a partition wall having an upper opening and a lower opening. Each chamber is provided with a perforated plate at the bottom on which a heat transfer medium containing a desulfurizing agent is placed and fluidized. The desulfurizing agent is transferred through the lower opening in the partition from the combustion chamber to the regeneration chamber and circulated by overflow through the upper opening back to the combustion chamber.
U.S. Pat. No. 4,407,206 teaches a process for partial combustion of coal containing more than 1% by weight sulfur and an organically bound calcium to sulfur ratio of at least 0.8 to 1 in which the coal is burned in a first combustion zone in the presence of an oxidizing agent, but under reducing conditions, producing a solid effluent and a gaseous effluent. After separation of the solid effluent from the gaseous effluent, the gaseous effluent is burned in a second combustion zone under oxidizing conditions.
U.S. Pat. No. 4,796,568 teaches a steam generating plant in which fuel, primarily coal, is burned in a fluidized bed of particulate material, such as limestone or dolomite, which serves as a sulfur absorbent. The vessel is divided into a combustion chamber and an ash chamber by an air distributor through which air for fluidization of the bed and combustion of the fuel is provided. The air distributor is provided with openings through which bed material can pass from the fluidized combustion chamber to the non-fluidized ash chamber.
U.S. Pat. No. 4,993,332 teaches a hybrid combustion system having a fluidized bed combustion portion and a pulverized coal combustion portion, the former including a fluidized bed chamber and the latter having at least one pulverized coal burner and an air recycling duct for recycling combustion air.
U.S. Pat. No. 4,815,418 teaches a boiler having two fluidized beds, pulverized coal being burned with air used as a fluidizing gas in the first fluidized bed and desulfurization of the resulting flue gases from the first fluidized bed which are used as fluidizing gases occurring in the second fluidized bed. See also U.S. Pat. No. 4,823,712 which teaches a bubbling bed, fluidized bed combustor capable of handling multiple fuels and U.S. Pat. No. 5,033,413 which teaches a fluidized bed combustion system having at least one circulating solids loop for fluidized bed particles.